1. Field of the Invention
This invention relates to a new process for the production of mercapto group-containing oligourethanes from organic polyisocyanates, organic polyhydroxyl compounds and mercapto-alkanols, certain acid esters of phosphoric acid being used as catalysts during reaction of the mercapto-alkanols; oligourethanes obtainable by this process and the use thereof as binders for oxidatively-hardenable coating and sealing compositions, as additives for epoxide resins and as cross-linking agents for plastics materials or plastics crude products containing olefinic double bonds.
2. Description of the Prior Art
Mercapto group-containing oligomers and polymers which may be oxidatively cross-linked and cured are known (see, for example, DE-OS No. 2,739,117, DE-OS No. 2,557,088, DE-AS No. 2,363,856 or DE-OS No. 3,121,384).
The important advantage of such binders over other categories of substances (such as oligourethanes having terminal NCO groups) resides, inter alia, in the fact that they also cure in the presence of water which may be added by moist pigments or additives or from the atmosphere, and form bubble-free polymers.
Furthermore, oligourethanes containing free NCO groups which are used as binders for sealing and coating compositions have several advantages besides the disadvantage of forming bubbles during curing with water, i.e., atmospheric moisture. They are suitable, for example, for the production of compositions which are particularly resistant to weathering and have long lasting elasticity, high toughness, good resistance to abrasion and stability towards solvents and other chemical influences.
Therefore, there have been many attempts to combine the valuable properties of the polyurethanes with the curing mechanism of mercapto groups. Oligourethanes having terminal mercapto groups are described in particular in DE-OS No. 1,770,259 and DE-AS No. 1,645,500. They are produced by firstly reacting a polyol with excess aromatic diisocyanate to form a prepolymer containing NCO groups and then urethanizing this prepolymer with an SH-group-containing alcohol. Oligourethanes containing terminal mercapto groups which cure oxidatively in bubble-free manner without foaming, even in the presence of moisture, are obtained in this way according to DE-OS No. 1,770,259 and DE-AS No. 1,645,500.
DE-OS No. 3,121,384 also describes oligourethanes having terminal mercapto groups. However, in contrast to DE-OS No. 1,770,259 and DE-AS No. 1,645,500, 1-isocyanatomethyl-1,3,3-trimethyl-5-isocyanatocyclohexane is used as isocyanate component in this case. Coating and sealing compositions which are characterized in particular by high light-fastness in addition to excellent stability to alkalies and heat are therefore obtained according to DE-AS No. 3,121,384.
Although the products described in the above-mentioned earlier publications basically represent valuable binders for sealing and coating compositions, they are accompanied to a greater or lesser extent by the disadvantage of a viscosity which is too high for many applications so that solvent-free use is often possible only when using considerable quantities of plasticizers. A certain advance was achieved with the systems in DE-OS No. 3,121,384, but a further reduction in viscosity of the products described in this earlier publication is still desirable.
It has now surprisingly been found that it is possible to reduce the viscosity of mercapto group-containing oligourethanes by producing the products by reacting mercapto-alkanols with organic polyisocyanates and sub-equivalent quantities of organic polyhydroxyl compounds or with NCO-prepolymers in the presence of certain acid esters of phosphoric acid described in more detail below.